The concept is encouraging

President Bush and Russian President Vladimir Putin may have found a way to avoid an imminent head-on collision over the 30-year-old Anti-Ballistic Missile Treaty. Their idea is to tie Bush's plan to test and deploy a missile defense system to Putin's proposal to reduce substantially each side's nuclear missile stockpile.

The two leaders announced the concept after meeting on Sunday in Italy. They are a long way from reaching an agreement, but the concept is encouraging.

A few weeks ago, Putin threatened to tear up other arms treaties and put multiple warheads on existing missiles if Bush proceeded with a missile defense system. Now, he says, that may not be necessary, creating room for compromise.

Bush, who hasn't retreated from his plan to test a defense system whether Russia likes it or not, must show equal flexibility and put off hasty moves, such as installing radar in violation of the ABM Treaty.

An agreement with Russia would help to placate America's European allies, who are worried about a renewed arms race. But talks would not include China, which, far more than Russia, believes its long-range fleet of only 20 missiles could be neutralized by a missile defense system.

Any effort by China to expand its missiles could set off an arms race that would ensnare Pakistan and India, which also have nukes. Proliferation in Asia could pose the biggest danger and unintended consequence of a missile defense system.

Putin may believe he has more to gain by compromising on missile defense and keeping on good terms with the United States than engaging in an arms race his country can't afford. The United States has about 7,000 strategic nuclear weapons and Russia has about 6,000, but many of Russia's are deteriorating, and Russia can't keep them up.

Under the signed-but-still-unratified START II treaty, both nations are supposed to cut the stockpile to between 3,000 and 3,500. Putin has suggested lowering that number to as few as 1,000 warheads a figure that's practical for Russia and better for the world's security, but one Bush may not be willing to match.

It's also unclear whether Russia would be willing to scrap the ABM Treaty or will insist on limiting changes to only minor modifications of it. Russia is less likely to acquiesce to Bush's grand plan for a space-based system with powerful lasers, which would undo the nuclear parity of the last 30 years.

Bush says that the ABM Treaty is a relic of the Cold War and that Russia has nothing to fear from a system designed to protect America from a sneak attack by a rogue nation.

Perhaps so, but Russia remains wary of America's intents, and significant doubts remain about the cost and feasibility of what Bush is proposing.

Persuading Russia to waive objections to a missile defense system would remove one major objection to it. But only one of many.

A deal to rid both nations of thousands of nuclear weapons, however, would be a triumph.